1. Field of the Invention
The present invention relates to a chromatic dispersion distribution measurement apparatus for measuring a chromatic dispersion distribution in an optical device to be measured, such as an optical fiber, and a method for the same.
2. Description of Related Art
In recent years, in order to satisfy the demands for high speed information communications, optical communication systems using optical fibers have been constructed. One of the factors in preventing the high speed signal transmission and the long transmission distance in the above optical communication systems, is the chromatic dispersion. The chromatic dispersion is a phenomenon caused by varying the speeds of lights transmitted in a medium, with the wavelength of the light. In the construction of the optical communication systems, it is necessary to grasp the chromatic dispersion characteristic in detail.
A chromatic dispersion distribution measurement apparatus for measuring the chromatic dispersion is shown in, for example, Japanese Patent Application Publication No. Tokukai-Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926 A2) In the publication, the chromatic dispersion distribution measurement apparatus measures the dispersion distribution in a longitudinal direction of a fiber to be measured, as follows. Two lights having different wavelengths from each other are inputted into the fiber to be measured. A specific wavelength component is extracted by an optical bandpass filter from a four-wave mixed light caused by the interaction between these two lights. A light having the extracted specific wavelength component is inputted into an Optical Time Domain Reflectometer (OTDR) to obtain the data to be used for calculating the chromatic dispersion distribution.
The chromatic dispersion distribution measurement apparatus described above measures the chromatic dispersion distribution in a specific wavelength. Therefore, it is impossible to measure a chromatic dispersion slope and a zero-dispersion wavelength (the wavelength at which the chromatic dispersion value is zero) because such measurements require extensive data over a considerable wavelength range. This makes it difficult to grasp the chromatic dispersion characteristics of the fiber to be measured.
In order to solve the above-described problems, an object of the present invention is to provide a chromatic dispersion distribution measurement apparatus and a method for the same, which interpolate the chromatic dispersion value at an optional wavelength to enable the measurement of the chromatic dispersion slope and the zero-dispersion wavelength.
That is, in accordance with the first aspect of the present invention, a chromatic dispersion distribution measurement apparatus, comprises:
a plurality of light sources selected from the group consisting of two variable wavelength light sources and at least three fixed-wavelength light sources which emit lights having different wavelengths from one another;
a chromatic dispersion distribution calculation unit for calculating a plurality of actual chromatic dispersion values in an optical device to be measured when a plurality of pairs of lights having different wavelengths from each other, which are emitted from the light sources, are inputted into the optical device to be measured;
a chromatic dispersion interpolation unit for interpolating an expected chromatic dispersion value between the actual chromatic dispersion values calculated by the chromatic dispersion distribution calculation unit, in order to obtain a chromatic dispersion distribution at an optional wavelength and at an optional transmission distance along the optical device to be measured;
a dispersion slope calculation unit for calculating a dispersion slope at the optional wavelength and at the optional transmission distance in accordance with the chromatic dispersion distribution; and
a zero dispersion wavelength calculation unit for calculating a zero dispersion wavelength at the optional transmission distance in accordance with the chromatic dispersion distribution.
In accordance with the second aspect of the present invention, a chromatic dispersion distribution measurement method, comprises:
calculating a plurality of actual chromatic dispersion values in an optical device to be measured when a plurality of pairs of lights having different wavelengths from each other are inputted into the optical device to be measured;
interpolating an expected chromatic dispersion value between the calculated actual chromatic dispersion values, in order to obtain a chromatic dispersion distribution at an optional wavelength and at an optional transmission distance along the optical device to be measured;
calculating a dispersion slope at the optional wavelength and at the optional transmission distance in accordance with the chromatic dispersion distribution; and
calculating a zero dispersion wavelength at the optional transmission distance in accordance with the chromatic dispersion distribution.
According to the first and the second aspects of the present invention, the dispersion slope at the optional wavelength and at the optional transmission distance and the zero dispersion wavelength at the optional transmission distance in the optical device to be measured can be calculated.
The chromatic dispersion interpolation unit may fit a plurality of the actual chromatic dispersion values calculated by the chromatic dispersion distribution calculation unit, into a predetermined chromatic dispersion equation.
The interpolating may be carried out by fitting a plurality of the calculated actual chromatic dispersion values into a predetermined chromatic dispersion equation.
As a predetermined chromatic dispersion equations, any equation can be used as long as a plurality of the chromatic dispersion values are fitted into it.
Therefore, from the chromatic dispersion values at the optional wavelengths, it is possible to grasp the chromatic dispersion distribution characteristics of the optical device to be measured.
The chromatic dispersion interpolation unit may employ a linear approximation by drawing a straight line between the actual chromatic dispersion values; and the dispersion slope calculation unit determines the dispersion slope by calculating a slope of the straight line.
The interpolating may be carried out by employing a linear approximation by drawing a straight line between the calculated actual chromatic dispersion values; and the dispersion slope calculating is carried out by calculating a slope of the straight line.
Therefore, the dispersion slope at the optional transmission distance in the optical device to be measured can be readily calculated with this technique, in particular, when the internal between the actual chromatic dispersion values is short.
In accordance with the third aspect of the present invention, a chromatic dispersion distribution measurement apparatus, comprises:
a chromatic dispersion value calculation unit for calculating a plurality of actual chromatic dispersion values in an optical device at a plurality of wavelengths; and
a chromatic dispersion interpolation unit for interpolating an expected chromatic dispersion value between the actual chromatic dispersion values calculated by the chromatic dispersion value calculation unit, in order to obtain a chromatic dispersion distribution as a function of wavelength.
The chromatic dispersion distribution measurement apparatus may further comprise a dispersion slope calculation unit for calculating a dispersion slope at an optional wavelength in accordance with the chromatic dispersion distribution.
The chromatic dispersion distribution measurement apparatus may further comprise a zero dispersion wavelength calculation unit for calculating a zero dispersion wavelength in accordance with the chromatic dispersion distribution.
The chromatic dispersion interpolation unit may fit the actual chromatic dispersion values calculated by the chromatic dispersion value calculation unit, into a predetermined chromatic dispersion equation.
The predetermined chromatic dispersion equation may be a Sellmeier""s formula.
The chromatic dispersion interpolation unit may employ a linear approximation by drawing a straight line between the actual chromatic dispersion values; and
the dispersion slope calculation unit may determine the dispersion slope by calculating a slope of the straight line.
In accordance with the fourth aspect of the present invention, a chromatic dispersion distribution measurement method, comprises:
calculating a plurality of actual chromatic dispersion values in an optical device at a plurality of wavelengths; and
interpolating an expected chromatic dispersion value between the calculated actual chromatic dispersion values, in order to obtain achromatic dispersion distribution as a function of wavelength.
The chromatic dispersion distribution measurement method may further comprise: calculating a dispersion slope at an optional wavelength in accordance with the chromatic dispersion distribution.
The chromatic dispersion distribution measurement method may further comprise: calculating a zero dispersion wavelength in accordance with the chromatic dispersion distribution.
The interpolating may be carried out by fitting the calculated actual chromatic dispersion values into a predetermined chromatic dispersion equation.
The predetermined chromatic dispersion equation may be a Sellmeier""s formula.
The interpolating may be carried out by employing a linear approximation by drawing a straight line between the calculated actual chromatic dispersion values; and
the dispersion slope calculating may be carried out by calculating a slope of the straight line.